1. Field of the Invention
The present invention relates, in general, to spacer grids used for placing and supporting fuel rods in nuclear reactor fuel assemblies and, more particularly, to a double strip mixing grid used in such nuclear reactor fuel assemblies and designed to effectively deflect and mix coolants together so as to improve the heat transferring effect between the fuel rods and the coolants, the mixing grid also designed to improve its fuel rod support performance so as to effectively protect the fuel rods from vibration and fretting failure of the fuel rods, and improve to effectively resist laterally directed forces acting thereon.
2. Description of the Prior Art
In typical nuclear reactors, a plurality of elongated nuclear fuel rods 125 are regularly and parallelly arranged in a fuel assembly 101 having a square cross-section. In such a case, for example, fourteen, fifteen, sixteen or seventeen fuel rods 125 are regularly arranged along each side of the square cross-section, thus forming a 14xc3x9714, 15xc3x9715, 16xc3x9716, or 17xc3x9717 array as shown in FIG. 1.
In order to place and support the fuel rods 125 within the nuclear fuel assembly 101, a plurality of spacer grids 110 are used. Each of such grids 110 is produced by intersecting a plurality of inner strips at right angles to form an egg-crate pattern, and welding the intersected strips at their intersections prior to encircling the periphery of the grid 110 with four perimeter strips. The top and bottom of the fuel assembly 101 are, thereafter, covered with top and bottom pallets 111 and 112. Therefore, the nuclear fuel assembly 101 is protected from any external loads acting on the top and bottom thereof. In the assembly, the spacer grids 110 and the pallets 111 and 112 are integrated into a single structure using a plurality of guide tubes 113. The elongated fuel rods 125, placed and supported within the fuel assembly 101 by the grids 110, are typically fabricated such that a fissionable fuel material, such as uranium core 114, is contained in a hermetically sealed elongated zircaloy tube, known as the cladding.
The above spacer grids 110 are each fabricated as follows. As best seen in FIG. 2, two sets of inner strips 115 and 116, individually having a plurality of notches at regularly spaced portions, are assembled with each other by intersecting the two sets of strips 115 and 116 at the notches, thus forming a plurality of four-walled cells. Each of the cells has four intersections 117. The assembled strips 115 and 116 are, thereafter, welded together at the intersections 117 prior to being encircled with the perimeter strips 118. A desired spacer grid 110 with such four-walled cells is thus fabricated.
As shown in FIG. 3, a plurality of positioning springs 119 and a plurality of positioning dimples 120 are integrally formed on or attached to the inner strips 115 and 116. In such a case, the springs 119 and the dimples 120 extend inwardly with respect to each of the four-walled cells. The dimples 120 are more rigid than the springs 119. In each of the four-walled cells, the positioning springs 119 force a fuel rod 125 against associated dimples 120, thus elastically positioning and supporting the fuel rod 125 at four points within each of the cells.
In such a typical nuclear fuel assembly 101, a plurality of spacer grids 110 having the above-mentioned construction are regularly and perpendicularly arranged along the axes of the fuel rods 125 at right angles, thus placing and supporting the fuel rods 125 within the assembly 101 at multiple points. That is, the spacer grids 110 form a multi-point support means for placing and supporting the fuel rods 125 within a nuclear fuel assembly 101.
In the typical nuclear fuel assembly 101, the positioning springs 119 elastically and lightly force the fuel rods 119 against the dimples 120 such that the fuel rods 125 are slidable on the support points of both the springs 119 and the dimples 120 when the fuel rods 125 are elongated due to thermal expansion or irradiation-induced growth of the fuel rods 125 within the fuel assembly 101.
When the fuel rods 125 are fixed to the spacer grids 110 at the support points of the grids 110, the fuel rods 125 may be bent at portions between the support points, thus undesirably reducing the intervals between the fuel rods 125 of the assembly 101 as shown in FIG. 4.
In some typical nuclear reactors using water as coolant, such as in the case of the nuclear reactors recently used in Korea, water receives thermal energy from the fuel rods 125 prior to converting the thermal energy into desired electric energy through a plurality of processes.
During an operation of a nuclear fuel assembly 101 of such a reactor, water or liquid coolant is primarily introduced into the assembly 101 through an opening formed on the core supporting lower plate of the reactor. In the assembly 101, the coolant flows upwardly through the passages, defined between the fuel rods 125, and receives thermal energy from the hot fuel rods 125.
The sectioned configuration of the coolant passages formed in the fuel assembly 101 is shown in FIG. 4.
In a conventional nuclear reactor, the amounts of thermal energy, generated from different portions of a nuclear fuel assembly 101, are not equal to each other. Since the fuel assembly 101 has a rectangular configuration, with a plurality of elongated, parallel fuel rods 125 closely set within the assembly 101 while being spaced apart from each other at irregular intervals, the temperature of coolant flowing around the fuel rods 125 is variable in accordance with positions of coolant currents relative to the rods 125.
That is, the amount of thermal energy received by water flowing around the corners 123 of each four-walled cell-is less than that received by water flowing around the fuel rods 125. The coolant passages of typical fuel assemblies 101 thus undesirably have low temperature regions.
Such low temperature regions reduce the thermal efficiency of the nuclear reactor. The coolant passages of the fuel assemblies 101 may also have partially overheated regions at positions adjacent to the fuel rods 125 having a high temperature. Such partially overheated regions deteriorate soundness of the assemblies 101.
In order to prevent such partially overheated regions from existing in a nuclear fuel assembly, it is necessary to design the spacer grid such that a uniform temperature distribution is formed in the fuel assembly. The grid must be also designed to effectively deflect and mix the coolant within the fuel assembly. Such effectively mixed coolant makes uniform the increase in enthalpy and maximizes the core output.
Typical examples of such designed spacer grids are disclosed in Korean Patent Publication Nos. 91-1978 and 917921.
In the spacer grids disclosed in the above-mentioned Korean patents, so-called xe2x80x9cmixing bladesxe2x80x9d or xe2x80x9cvanesxe2x80x9d are attached along the upper edges of the intersecting strips of each grid, and are used for mixing coolants within- the fuel assembly. That is, the mixing blades or vanes allow the coolant to flow laterally, in addition to normally flowing in an axial direction, as shown in FIG. 3, and so the coolants are effectively, mixed with each other between the coolant channels and between the lower temperature regions and the partially overheated regions of the fuel-assembly.
In the prior art, the techniques for mixing the coolants with each other between the coolant channels and between the lower temperature regions and the partially overheated regions of the fuel assembly using such mixing blades or vanes are classified into two types: the first technique using large-scaled mixing blades for creating a lateral flow of coolant and the second technique using vanes provided at the intersections for creating a swirling flow of coolant. In the first technique, the coolants, axially flowing along the elongated fuel rods within a fuel assembly, partially collide against the large-scaled mixing blades so as to flow laterally, in addition to normally flowing in the axial direction. In the second technique, a plurality of vanes are provided at the intersections of the spacer grid for creating the, swirling flow of coolants.
However, the conventional two techniques for mixing the coolants with each other between the channels using such mixing blades or vanes are problematic in that the pressure of the coolants is undesirably reduced in inverse proportion to the expected coolant mixing effect, and so the two techniques are. undesirably limited in their coolant mixing effects.
That is, the wake stream, disturbing the main flow of the coolants, or the vortex flow of the coolants, generated at. positions around the bent portions of the mixing blades or vanes, is enhanced in proportion to the size or the bending angle of the mixing blades or vanes, which is enlarged for enhancing the lateral flow or swirling motion of the coolants within a nuclear fuel assembly. Therefore, the pressure of the coolants in such a case is reduced to limit the enhancement of the desired lateral flow or the desired swirling motion of the coolants. This limits the size and bending angles of the mixing blades or the vanes, and limits the thermal hydraulic efficiency of the nuclear fuel assembly.
In addition, a double strip mixing, grid comprising two sets of intersecting inner strips individually consisting of two sheets specifically deformed and integrated together to define coolant channels between them, has been proposed. In this double strip mixing grid, the upper or lower portion of each channel is inclined relative to the axes of the fuel rods at an angle of inclination, thus producing a swirling motion of coolant at the inlet and outlet of the channels. Such a swirling motion of coolant improves the heat transferring effect between the fuel rods and the coolant within a nuclear fuel assembly.
Such a double strip mixing grid having the two sets of intersecting inner strips is referred to U.S. Pat. No. 4,726,926, Korean Patent No. 265,027, and U.S. Pat. No. 6,130,927.
The above U.S or Korean grids, designed to form a lateral flow of coolants or to deflect and mix the coolants within a nuclear fuel assembly, are somewhat advantageous in that they more effectively mix the coolants and improve the heat transferring effect between the fuel rods and the coolants within a nuclear fuel assembly. However, such a conventional double strip mixing grid is problematic in that the lateral flow or mixing of coolants regrettably vibrates the elongated, parallel, closely spaced fuel rods within the assembly.
In the conventional spacer grids for nuclear fuel assemblies with the fuel rods 125 supported by both the positioning springs 119 and the positioning dimples 120 within the four-walled cells of the grids 110, the fuel rods 125 during an operation of a nuclear fuel assembly 101 briefly and periodically interfere with the intersecting strips of the grids due to vibrations caused by the lateral flow of coolants. When the fuel rods 125 are so vibrated over a lengthy period of time, the claddings of the fuel rods 125 are repeatedly and frictionally abraded at their contact parts at which the fuel rods 125 are brought into contact with the springs and dimples of the grids. The claddings are thus reduced in their thickness so as to be finally perforated at said contact parts. Such an abrasion of the fuel rods is so-called fretting wear in the art.
Detailed description of such a fretting wear may be referred to Korean Patent Publication No. 94-3799.
The laterally directed force caused by the mixing blades of the grids is in proportion to the coolant mixing effect, and directly affects the heat transferring effect of nuclear reactors. However, such a laterally directed force of the mixing blades also proportionally increases the amplitude of vibration of the fuel rods. This may cause damage to the fuel rods.
The important factors necessary to consider while designing the grids for use in nuclear fuel assemblies are improvement in both the fuel rod supporting function of the grids and the buckling strength capable of resisting such a laterally directed force acting on the grids.
During an operation of a nuclear reactor, the fuel assemblies may be vibrated laterally due to load acting on the assemblies and this causes interference between the assemblies. Therefore, the grids of the fuel assemblies may be impacted due to such interference between the fuel assemblies as disclosed n U.S. Pat. No. 4,058,436.
In the prior art, the grid""s buckling strength, resisting a lateral load acting on the grid, is undesirably reduced since the grid strips have to be partially removed, for example, through a stamping process at a plurality of portions so as to form positioning springs and dimples of the grid within a fuel assembly. Such cut-away portions reduce the effective cross-sectional area of the grid capable of resisting impact, and reduce the buckling strength of the grid.
In a grid disclosed in U.S. Pat. No. 5,243,634, the positioning springs are individually integrated with an associated grid strip at one point, thus forming a cantilever structure. Such a cantilever spring is more flexible than a simple spring, which is integrated with a grid strip at opposite ends thereof.
In the mixing grid disclosed in the above-mentioned U.S. Pat. No. 4,726,926, the deformed portions, provided on the sheets of the intersecting grid strips for forming the channels for coolant, collaterally act as channel-shaped positioning springs used for placing and supporting the fuel rods within the four-walled cells of the grid.
Since the sheets of the strips are not cut away, but deformed to form such channel-shaped springs, flexibility of such channel-shaped springs is exceedingly-less in comparison with the above-mentioned cantilever springs, thus failing to provide desired flexibility expected by conventional positioning springs. The channel-shaped springs thus act as dimples rather than springs. Therefore, the above-mentioned mixing grid, having such channel-shaped dimples and supporting the elongated fuel rods using only the channel-shaped dimples without having springs, is problematic in that said dimples may cause the fuel rods to be undesirably bent when the fuel rods are elongated due to the irradiation-induced growth during an operation of the reactor. In addition, this mixing grid is reduced in its elastic range, wherein the grid effectively and elastically supports the fuel rods in the fuel assembly, and so the mixing grid may be apt to. lose its spring function during a grid manufacturing process, during a fuel rod installing process, or when the fuel rods are elongated due to the irradiation-induced growth during an operation of the reactor. In such a case, the mixing grid may undesirably lose its function of effectively placing and supporting the fuel rods within a nuclear fuel assembly or restricting undesired vibration of the fuel rods.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a double strip mixing grid for nuclear reactor fuel assemblies, which has mixing blades used in a conventional single strip spacer grid, in addition to having coolant channels used for effectively. mixing the low temperature coolant with the high temperature coolant in the same manner as disclosed in Korean patent No. 265,027, thus having advantages expected from both the mixing blades of the conventional single strip spacer grid and the coolant channels of the conventional double strip mixing grid, and remarkably improving thermal efficiency of the fuel assemblies; and which has an enhanced spring function of supporting the fuel rods within the fuel assemblies, thus almost completely preventing fretting wear of the fuel rods caused by hydraulic vibration of the fuel rods within the nuclear fuel assemblies.
Another object of the present invention is to provide a double strip mixing grid for nuclear reactor fuel assemblies, of which the intersecting strips are slightly reduced in thickness, but are not cut away at any portion, thus maintaining a desired effective sectional area thereof and thereby having a desired buckling strength capable of effectively resisting lateral load acting thereon, and which improves the thermal hydraulic strength and mechanical strength of the fuel assemblies.
A further object of the present invention is to provide a double strip mixing grid for nuclear reactor fuel assemblies, of which the intersecting strips have a reduced thickness and a substantially longer interval between their fuel rod support points, and which maximizes the elastic range of the positioning springs, and increases the number of fuel rod contact points and fuel rod contact areas, thus more effectively placing and supporting the fuel rods within the fuel assembly, and in which the fuel rods are not supported by only the positioning springs; and so the elastic displacement of the fuel rods is more effectively supported by the grid of this invention in comparison with a known grid using only such positioning springs, thus overcoming the problems experienced in the nozzle-type positioning springs or dimples having an excessively high strength.
In order to accomplish the above objects, the present invention provides a double strip mixing grid for nuclear reactor fuel assemblies, comprising a plurality of inner double strips, each fabricated by integrating two thin sheets together into a single structure having a plurality of coolant channels and intersecting each other at a predetermined angle to form a desired mixing grid having a plurality of four-walled cells, wherein each of the strips is partially cut away at predetermined portions along its top edge to form nozzles of the coolant channels.
In the double strip mixing grid of this invention, each of the intersecting double strips is fabricated by integrating two thin sheets together into a single structure having the coolant channels. The cross-sectional area of each of the coolant channels of each strip consisting of the two thin sheets is gradually increased from its bottom end, having a predetermined cross-sectional area, and is maximized at the middle portion of the -channel supporting the fuel rod, and is gradually reduced to become zero at the top end of the strip.
The top end of the strip is partially cut away at a position around the outlet end of each channel, thus forming an outlet nozzle of the channel. The coolant flows from the nozzle of the channel, and allows the coolant flowing outside the channels to smoothly flow without forming any vortexes.
In the double strip mixing grid of this invention, the coolant flowing from the nozzles of the channels collides. against the mixing blades provided on the strip at positions around the nozzles, and so a desired swirling flow and/or a desired lateral flow of coolant is created within the fuel assembly. However, the mixing blades also form undesired vortexes in the coolant.
In the mixing grid of this invention, the coolant flowing outside the channels is mixed with the coolant flowing from the channels prior to colliding against the mixing blades. This mixing grid thus allows the coolant to smoothly flow within the fuel assembly without forming such undesired vortexes in the coolant.
In the double strip mixing grid of this invention, the cross-sectional area of each of the coolant channels of each strip is gradually increased from its bottom end, and is maximized at its middle portion supporting the fuel rod, and is gradually reduced in a direction toward the top end of the strip. In order to prevent the sheets of the strips from having an excessively high strength at positions around the middle portions of the channels supporting the fuel rods, a vertical slot is formed on each sheet of the strip at a position around each channel, thus improving the spring function of the fuel rod contact portions of the double strips. Due to the slots, the double strips of the grid more effectively support the fuel rods while holding the rods at positions around the slots, and so the mixing grid of this invention enlarges its fuel rod contact area, and effectively protects the fuel rods from a fretting corrosion.
In the double strip mixing grid of this invention, each of the strips has a thickness ranging from 0.25 mm to 0.40 mm, with each of the channels having a width ranging from 7 mm to 10 mm.
In addition, the intersecting inner strips of the mixing grid according to this invention may be preferably and continuously welded together at their intersections through a continuous welding process or a seaming process, in addition to a conventionally performed alternate spot welding process by which big weld nugget is normally formed at the intersection. Therefore, it is possible to improve the mechanical strength of the mixing grid, such as the impact strength or bending strength of the grid.